CHEMISTRY COMMUNITY

Why can we not add the cell potential of elementary steps?

I think what you're referring to is that when you have to calculate the E value for a reaction like this: Ti⁺⁴ + 2e- -> Ti⁺², you cannot simply add the E values for the reactions Ti⁺⁴ + e- -> Ti⁺³ and Ti⁺³ + e- -> Ti⁺².

The details about why this happens aren't too important, but this is a good general rule: If the reaction you're calculating the E value for has electrons in it, you cannot simply add the E values from the 2 reactions needed to make that reaction. If the reaction you're calculating the E value for does not have electrons in it, you can simply add the E values. :)

Also, cell potential only refers to electrochemistry, more specifically redox reactions. In kinetics, elementary steps are used to find the reaction mechanism that matches the rate law... they usually wont have a given cell potential as these are usually not reduction or oxidation reactions.